Method of manufacturing a short-channel FET with Schottky-barrier source and drain contacts

ABSTRACT

The present invention Is a fabrication method for a short-channel Schottky-barrier field-effect transistor device. The method of the present invention includes introducing channel dopants into a semiconductor substrate such that the dopant concentration varies in the vertical direction and is generally constant in the lateral direction. A gate electrode is formed on the semiconductor substrate, and source and drain electrodes are formed on the substrate to form a Schottky or Schottky-like contact to the substrate.

FIELD OF THE INVENTION

The present invention relates generally to field of metal oxidesemiconductor field effect transistors (MOSFETs), and has specificapplication to the fabrication of these devices in the context of anintegrated circuit (IC).

BACKGROUND OF THE INVENTION

Since the invention of the transistor in the late 1940s, tremendousadvances have been made in the field of microelectronics. Currenttechnology allows for the cost-effective fabrication of integratedcircuits (ICs) with over 100 million components—all on a piece ofsilicon roughly 10 mm on a side. The one billion transistor IC will becommercially available within a few years. The desire for greaterfunctionality and performance at less cost per IC drives several trends.

First, functionality drives IC transistor counts up. Second, thetransistors themselves are being reduced in size so as to achievegreater packing density and, very importantly, to improve theirperformance. As far as performance is concerned, the key parameter forMetal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETs, thedominant transistor technology of the day) is the channel length. Thechannel length (L) is the distance that charge carriers must travel topass through the device, and a reduction in this length simultaneouslyimplies higher current drives, reduced parasitic resistances andcapacitances and improved high-frequency performance. A commonfigure-of-merit is the power-delay product, and this generalized measureof transistor performance improves as the cube of the inverse of thechannel length (1/L³). This explains the tremendous incentive that ICmanufacturers have to reduce the channel length as much as manufacturingcapabilities will allow.

For digital applications, MOS transistors behave like switches. When‘on’, they drive relatively large amounts of current, and when turned‘off’ they are characterized by a certain amount of leakage current. Aschannel lengths are reduced, drive currents increase, which isbeneficial for circuit performance as stated above. However, leakagecurrents increase as well. Leaky transistors contribute to quiescentpower dissipation (the power dissipated by an IC when idle) and inextreme cases can affect the transfer of binary information duringactive operation. Device designers therefore have good reason to keepleakage currents low as channel lengths are reduced.

MOS transistor leakage currents are traditionally controlled byintroducing controlled amounts of impurities (dopants) into the channelregion of the device, and by tailoring the source/drain lateral andvertical doping distributions. Although these approaches are effectivein shoring up the potential barrier internal to the MOS transistor andtherefore reducing the leakage current, they can also contribute todegraded drive current and increased parasitic capacitance—the veryitems that channel length reduction is meant to improve. Furthermore,depending on exactly how in the manufacturing process the channel andtailored source/drain dopants are introduced, the manufacturing cost canbe affected significantly. Given traditional MOS transistor design andarchitecture, there are only limited solutions to the trade-off betweendrive current, leakage current, parasitic capacitance and resistance,and manufacturing complexity/cost.

The present invention offers a new relationship between these competingrequirements, and makes possible MOS devices with characteristics thatare not achievable with traditional (impurity doped) MOS architectures.The use of metal for the source and drain and a simple, uniformlyimplanted channel dopant profile provides for improvements to devicecharacteristics in terms of reduced parasitic capacitance, reducedstatistical variations in these characteristics (especially as thechannel length is decreased) and reduced manufacturing cost andcomplexity.

DESCRIPTION OF THE PRIOR ART Doping Profiles

Previous generations of MOS transistors have relied on laterallyuniform, and vertically non-uniform channel doping profiles to controldrain-to-source leakage currents. See Yuan Taur, “The IncredibleShrinking Transistor”, IEEE SPECTRUM, pages 25-29(www.spectrum.ieee.org, ISSN 0018-9235, July 1999). FIG. 1 illustratesan exemplary long-channel conventional MOS device (100) that comprisesan impurity doped source (101), an impurity doped drain (102), aconventional MOS type gate stack (103), and a laterally uniform channeldoping profile (104) in the substrate to assist in the control ofsource-to-drain leakage currents. Devices are electrically isolated fromeach other via a field oxide (105). Such channel dopant profiles arecommon in devices with channel lengths down to approximately200nanometers (nm).

However, as device channel lengths have been reduced into the 100 nmregime the literature teaches that channel doping profiles that arenon-uniform in both the lateral and vertical directions are required.Referencing FIG. 2, the exemplary short-channel MOS device (200) hassome elements similar to the long-channel MOS device (100). Thestructure comprises a conventional impurity doped source (201) and drain(202) as well as a conventional MOS gate stack (203) (width<˜100 nm,corresponding to the channel length L). The structure further comprisesshallow, impurity doped extensions for the source (208) and drain (209)electrodes which are used in conjunction with drain (206) and source(207) pocket doping as well as conventional channel doping (204) tocontrol source to drain leakage currents. Source and drain electrodes(201) and (202) and their respective extensions (208) and (209) (thecombination of all four of which comprise the tailored source/draindoping profile) are all of the same doping polarity (either N-type orP-type) and are of the opposite polarity from the channel (204) andpocket doping elements (206) and (207). Again, a field oxide (205)electrically isolates devices from each other.

In his paper entitled “25 nm CMOS Design Considerations” (1998 IEDMTechnical Digest, page 789), Yuan Taur states

“. . . an optimized, vertically and laterally non-uniform dopingprofile, called the super-halo, is needed to control the short channeleffect.”

A similar statement has been made in the IEEE Spectrum magazine:

“. . . in the 100 to 130 nm lithography generation, an optimallytailored profile that is both vertically and laterally non-uniform(Super-Halo) is need to control [short channel effects].”

See Linda Geppert, “The 100-Million Transistor IC”, IEEE SPECTRUM, pages23-24 (www.spectrum.ieee.org, ISSN 0018-9235, July 1999).

Furthermore, virtually all the prior art that discusses device designfor channel lengths less than 200 nm states or implies that channeldoping profiles that are highly non-uniform in both the lateral andvertical directions are required for adequate control of drain-to-sourceleakage currents. For example, Hargrove in his paper “High-Performancesub 0.08um CMOS with Dual Gate Oxide and 9.7

Inverter Delay” (1998 IEDM, page 627) states

“In order to achieve optimal device performance . . . strong haloscoupled with shallow junctions are required”.

The prior art is virtually unanimous in its statement that laterally andvertically non-uniform doping profiles, in the form of laterallynon-uniform channel dopants and shallow source/drain extensions, arerequired for adequate control of short channel effects.

Pocket/Halo Implants

Laterally non-uniform channel doping profiles are almost exclusivelyintroduced after the gate electrode has been defined and is in place.With the gate serving as an implant mask, dopants of the same type asthose already in the substrate are introduced into the channel regionsadjacent to the gate electrode's edges via ion-implantation. Asmentioned previously, these are often referred to as “pocket” or “halo”implants. See Yuan Taur, “The Incredible Shrinking Transistor”, IEEESPECTRUM, page 28 (www.spectrum.ieee.org, ISSN 0018-9235, July 1999).

While effective at reinforcing the electrostatic potential barrierbetween the source and drain (and therefore reducing leakage currents),Halo/Pocket implants along with shallow source/drain extensions (thetailored source/drain doping profiles mentioned previously) addcomplexity to the manufacturing process. At least two additionallithography steps, as well as the associated cleans, implants,metrology, etc. are required to implement these process steps. Aslithography is one of the most (if not the most) expensive processmodules in the production process, this is a significant increase inmanufacturing cost. Halo and Pocket implants as well as shallowsource/drain extensions can also add parasitic capacitance and a randomstatistical variation to device electrical characteristics.

Channel doping profiles for short channel Schottky MOS devices havereceived only very limited attention in the prior art. J. R. Tuckerdiscusses simulations done on very short channel SBMOS devices and onlymentions in passing that

“. . . some doping of the semiconductor channel region will be requiredin order to suppress (leakage) currents . . .”.

See J. R. Tucker, C. Wang, J. W. Lyding, T. C. Shen, G. C. Abeln,“Nanometer Scale MOSFETs and STM Patterning on Si”, SSDM 1994, pages322-324; J. R. Tucker, C. Wang, P. S. Carney, “Silicon Field-EffectTransistor Based on Quantum “Tunneling”, Applied Physics Letters, Aug.1, 1994, Vol. 65, No.5, pages 618-620. It is significant to note thatTucker does not discuss in what manner one might go about introducingchannel doping to suppress source-to-drain leakage currents.

Q. T. Zhao is the next author to explicitly address the issue of channeldoping to control leakage currents. His approach (uniform doping of thesubstrate to quite high levels (10¹⁷/cm³)) is well known to benon-optimal for short channel devices. Although he is successful inreducing leakage currents, he does so at the expense of increasedsource/drain-to-substrate capacitance. See Q. T. Zhao, F. Klinkhammer,M. Dolle, L. Kappius, S. Mantl, “Nanometer patterning of epitaxialCoSi₂/Si(100) for ultrashort channel Schottky barriermetal-oxide-semiconductor field effect transistors”, APPLIED PHYSICSLETTERS, Vol. 74 No. 3, 18January 1999, page 454.

W. Saitoh reports on a device built on SOI substrates but does notdiscuss substrate doping in this context. See W. Saitoh, S. Yamagami, A.Itoh, M. Asada, “35 nm metal gate SOI-P-MOSFETs with PtSi Schottkysource/drain”, Device Research Conference, Jun. 28-30, 1999, SantaBarbara, Calif. Paper II.A.6, page 30.

C. Wang mentions the use of “a layer of fully-depleted dopants beneaththe active region” and “preimplanting a thin subsurface layer of fullydepleted dopants” to control leakage currents, but does not describe thelateral uniformity or lack thereof of the doping profile, or how onemight go about producing the “layer”. See C. Wang, John P. Snyder, J. R.Tucker, “Sub-40 nm PtSi Schottky source/drain metal-oxide-semiconductorfield-effect-transistors”, APPLIED PHYSICS LETTERS, Vol. 74 No. 8, Feb.22, 1999, pages 1174; C. Wang, John P. Snyder, J. R. Tucker, “Sub-50 nmPtSi Schottky source/drain P-MOSFETs”, Annual Device Research ConferenceDigest 1998, pages 72-73.

SUMMARY

Given the literature on substrate doping profiles for conventional shortchannel MOS transistors and the scant work on channel doping profilesfor short channel Schottky MOS devices, the proposed invention offers anovel and non-obvious approach with many advantages over the currentstate-of-the-art.

OBJECTS OF THE INVENTION

Accordingly, the objects of the present invention are (among others) tocircumvent the deficiencies in the prior art and affect one or more ofthe following objectives:

1. Provide a system and method to permit MOSFETs to be fabricated withshort channel lengths with less cost, higher performance and bettertolerances than current fabrication technologies.

2. Reduce parasitic bipolar operation in integrated MOSFETs, thusreducing the potential for latchup and other anomalous behavior.

3. Provide for MOSFET devices that have in some circumstances a higherdegree of radiation hardness.

While these objectives should not be understood to limit the teachingsof the present invention, in general these objectives are achieved bythe disclosed invention that is discussed in the following sections.

BRIEF SUMMARY OF THE INVENTION Overview

Referencing FIG. 3, an exemplary embodiment of the present invention(300) consists simply of a conventional MOS gate stack (303) (gateelectrode on silicon dioxide on a silicon substrate), metal source (301)and/or drain (302) electrodes, and channel dopants (304) that varysignificantly in the vertical direction but not in the lateraldirection. A field oxide (305) electrically isolates devices from eachother.

The Schottky (or Schottky-like) barriers (307, 308) that exist along theinterface of the corresponding metal source/drain (301, 302) and thesilicon substrate (306) act as an inherent pocket or halo implant anddoes so without added parasitic capacitance. It also eliminates the needfor shallow source/drain extensions as the metal source/drain is by itsnature shallow and highly conductive. Significant reductions inmanufacturing complexity are therefore achieved by simultaneouselimination of the halo/pocket implants and the source/drain extensions.These are also major advantages over conventionally-architected-channelMOS devices.

Due to the atomically abrupt nature of the Schottky barrier and the veryconsistent and repeatable magnitude of this barrier, two sources ofstatistical variation which are endemic to conventional MOS devices arevirtually eliminated. The random statistical nature of dopantintroduction via ion-implantation in conventional devices producessignificant variations in the position and magnitude of implanteddopants. This is true for both the halo/pocket and source/drain dopants.The result is a certain amount of random variation in device parameterssuch as channel length (L), drive current, and leakage current. Thesevariations make circuit design more difficult and contribute tomanufacturing cost via yield loss due to ICs that do not meetperformance specifications. The problem becomes more severe as channellengths are reduced due to the smaller effective volume of silicon perdevice, and therefore less averaging to smooth away statisticalvariations.

Because the metal source/drain (which replaces the conventional impuritydoped source/drain) has a natural, very consistent and atomically abruptSchottky barrier (307, 308) with the silicon substrate (306) whoseposition and magnitude are independent of channel length, and becausethis barrier essentially plays the role of the halo/pocket implant(making these implants unnecessary), statistical variations due torandom placement of atoms during the source/drain and halo/pocketimplants are essentially eliminated. This fact remains true and evenbecomes more true as the channel length is reduced.

Another benefit of the metal source/drain MOS architecture is theunconditional elimination of the parasitic bipolar gain. The parasiticbipolar gain is a direct result of using opposite doping types for thesource/drain and substrate regions, and can result in latch-up and otherdeleterious effects. When the source/drain electrodes are constructed ofmetal, this parasitic gain is eliminated. This makes the metalsource/drain architecture ideal for (among other things) high-radiationenvironments.

General Advantages

The present invention typically provides the following benefits ascompared to the prior art:

1. Reduction in manufacturing complexity. Pocket/Halo implants andshallow source/drain extensions are not needed.

2. Reduction in capacitance due to absence of pocket/halo implants.

3. Reduction in random/statistical variations of device electricalcharacteristics due to absence of pocket/halo implants and source/drainextensions, and the use of metal for the source and drain.

4. Unconditional elimination of the parasitic bipolar gain andassociated latchup.

5. Increased radiation hardness as compared to conventional MOSstructures.

The above list of advantages should not be interpreted as to limit thescope of the present invention. However, one skilled in the art willrecognize a plethora of application opportunities for the presentinvention teachings given the above-mentioned list of general advantagesthat are potentially available.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the advantages provided by the invention,reference should be made to the following detailed description togetherwith the accompanying drawings wherein:

FIG. 1 illustrates a prior art long channel, impurity doped source/draindevice;

FIG. 2 illustrates a prior art short channel, impurity dopedsource/drain device with pocket implants and source/drain extensions;

FIG. 3 illustrates an exemplary embodiment of the present invention asapplied to a short channel, metal source/drain device without pocketimplants;

FIG. 4 illustrates an exemplary embodiment of the present inventionprocess fabrication step using an implanted silicon substrate withapproximately 200 Å screen oxide;

FIG. 5 illustrates an exemplary embodiment of the present inventionprocess fabrication step using a patterned in-situ doped silicon film onthin gate oxide;

FIG. 6 illustrates an exemplary embodiment of the present inventionprocess fabrication step using a formation of thin oxide sidewalls, andexposure of silicon in the gate, source and drain areas;

FIG. 7 illustrates an exemplary embodiment of the present inventionprocess fabrication step using a metal deposition and silicidationanneal.

FIG. 8 illustrates an exemplary embodiment of the present inventionprocess fabrication step using a removal of unreacted metal from thesidewalls;

FIG. 9 illustrates an exemplary embodiment of the present inventionprocess fabrication resulting structure. In-situ phosphorous dopedsilicon, Erbium silicide and Indium channel implants are used for theN-type device for the gate electrode, source/drain electrodes andchannel doping respectively. The P-type devices make use of in-situBoron doped silicon, Platinum Silicide and Arsenic channel implants. Thechannel dopant concentrations vary significantly in the verticaldirection but not in the lateral direction. Gate lengths are typically<100 nm, but may be longer;

FIG. 10 illustrates an exemplary general system process flowchartshowing a method of producing superior MOSFET devices;

FIG. 11 illustrates an exemplary detailed system process flowchartshowing a method of producing superior MOSFET devices.

DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

Embodiments are Exemplary

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetailed preferred embodiment of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiment illustrated.

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiments, wherein these innovative teachings are advantageouslyapplied to the particular problems of a MOSFET DEVICE SYSTEM AND METHOD.However, it should be understood that these embodiments are onlyexamples of the many advantageous uses of the innovative teachingsherein. In general, statements made in the specification of the presentapplication do not necessarily limit any of the various claimedinventions. Moreover, some statements may apply to some inventivefeatures but not to others. In general, unless otherwise indicated,singular elements may be in the plural and visa versa with no loss ofgenerality.

DEFINITIONS

Throughout the discussion in this document the following definitionswill be utilized:

System Blocks/Procedural Steps Not Limitive

The present invention may be aptly described in terms of exemplarysystem block diagrams and procedural flowcharts. While these items aresufficient to instruct one of ordinary skill in the art the teachings ofthe present invention, they should not be strictly construed as limitingthe scope of the present invention. One skilled in the art will be awarethat system block diagrams may be combined and rearranged with no lossof generality, and procedural steps may be added or subtracted, andrearranged in order to achieve the same effect with no loss of teachinggenerality. Thus, it should be understood that the present invention asdepicted in the attached exemplary system block diagrams and proceduralflowcharts is for teaching purposes only and may be reworked by oneskilled in the art depending on the intended target application.

Predetermined Regions

Throughout the following discussion the term ‘predetermined region’ willbe defined to encompass the area centered at the heart of the activedevice (MOSFET). Thus, all the process steps mentioned in the context ofa MOSFET will create a gate, source/drain, and/or channel dopingprofiles as well as other structures in an area proximal to thepredetermined regions or heart of the active device. The presentinvention places no restrictions on what occurs outside this context,far from the heart of the active device.

It should be noted that while the predetermined region will generally bespoken of in terms of a MOSFET device, this in no way limits the scopeof the present invention. One skilled in the art will recognize that anydevice capable of regulating the flow of electrical current may beconsidered to have a predetermined region in proximity to its activecurrent carrying region.

MOSFET Not Limitive

The present invention is particularly suitable for use with MOSFETsemiconductor devices, but the use of the present teachings is notlimited to this particular application. Other semiconductor devices,whether integrated or not, may be applied to the present inventionteachings. Thus, while this specification speaks in terms of ‘MOSFET’devices, this term should be interpreted broadly to include any devicefor regulating the flow of electrical current having a conductingchannel that has two or more points of electrical contact.

Channel Length Not Limitive

The present invention is particularly suitable for use in situationswhere short channel length MOSFETs are to be fabricated, especially inthe range of channel lengths <100nm. However, nothing in the teachingsof the present invention limits application of the teachings of thepresent invention to these short channel length devices. Advantageoususe of the teachings of the present invention may be had with channellengths of any dimension.

Dopants Not Limitive

Throughout the discussion herein there will be examples provided thatutilize various dopant technologies in regards to MOSFET devicefabrication. These dopants are only illustrative of a specificembodiment of the present invention and should not be interpreted to beLimitive of the scope of teachings within the current invention.

Note, however, that the present invention specifically anticipates theuse of impurity atoms are selected from the group consisting of Arsenic,Phosphorous, Antimony, Boron, Indium, and/or Gallium as being within thescope of the teachings of the present invention.

Device Type Not Limitive

One skilled in the art will readily realize that the present inventionis not limited in scope to either N-type or P-type devices but may beused with either or both device types.

Source/Drain Not Limitive

Throughout the discussion herein there will be examples provided thatmake reference to ‘source’ and ‘drain’ connections in regards to MOSFETdevice fabrication. One skilled in the art will recognize that in anygiven MOSFET configuration the nomenclature surrounding these contactsmay be swapped without loss of generality, so that the ‘source’ may beinterchanged with the ‘drain’ contact with no loss in the scope of thepresent invention. Additionally, one skilled in the art will recognizethat while many preferred embodiments of the present invention may beused to fabricate both source and drain connections, there is norequirement that this be the case in actual practice. One, both, or noneof the source/drain connections on a given device in the context of anIC or the like may use the teachings of the present invention toadvantage.

Thus, the terms ‘source’ and ‘drain’ should be interpreted to includethe variants ‘drain’ and ‘source’ as well as ‘source or drain’ and‘source and drain’.

Metals Not Limitive

Throughout the discussion herein there will be examples provided thatmake reference to metals in regards to MOSFET device fabrication. Thepresent invention does not recognize any limitations in regards to whattypes of metals may be used in affecting the teachings of the presentinvention. Thus, metals commonly used at the transistor level such astitanium, cobalt and the like are specifically anticipated, as well as aplethora of more exotic metals and other alloys. Nothing in thedisclosure limits the use of the invention with any particular metal oralloy. One skilled in the art will recognize that any conductiveinterconnecting material may be used with no loss of generality inimplementing the teachings of the present invention.

Note, however, that the present invention specifically anticipates theuse of source/drain electrodes formed from the group consisting of anyof Platinum Silicide, Palladium Silicide, Iridium Silicide, and/or therare-earth suicides as being within the scope of the teachings of thepresent invention.

Schottky Not Limitive

Throughout the discussion herein there will be examples provided thatmake reference to ‘Schottky’ barriers and like contacts in regards to ICfabrication. The present invention does not recognize any limitations inregards to what types of Schottky interfaces may be used in affectingthe teachings of the present invention. Thus, the present inventionspecifically anticipates these types of junctions to be created with anyform of conductive material.

Additionally, while traditional Schottky junctions are abrupt, thepresent invention specifically anticipates that in some circumstances aninterfacial layer may be utilized between the silicon substrate and theactual Schottky barrier metal. Thus, the present invention specificallyanticipates ‘Schottky-like’ junctions and their equivalents to be usefulin implementing the present invention. Furthermore, the interfaciallayer may comprise materials that have conductive, semi-conductive,and/or insulator-like properties.

Etching Technology Not Limitive

Throughout the discussion herein there will be examples provided thatmake reference to a variety of etching technologies utilized to removeoxide and/or metal in the IC fabrication process. The present inventiondoes not restrict the type of etching technology used to achieve theresults illustrated in the typical process flows. These etchingtechnologies are well known in the art.

Process/Method

One possible process flow for the fabrication of channel implanted,short channel (<100 nm) metal source/drain MOS devices (400) isillustrated in FIGS. 4-9. While this exemplary process flow is justexemplary of the broad teachings of the present invention, it will provevery instructive to one skilled in the art to teach the fundamentalconcepts of the present invention. This exemplary process flow may bedescribed as follows:

1. Referencing FIG. 4, starting with a silicon substrate (402) that hasmeans for electrically isolating transistors from one another, a thinscreen oxide (401) is grown (approximately 200 Å) to act as an implantmask. The appropriate channel dopant species (403) (for example Arsenicand Indium for P-type and N-type devices respectively) is thenion-implanted through the screen oxide to a pre-determined depth in thesilicon (approximately 1000 Å or so).

2. Referencing FIG. 5, the screen oxide is then removed in hydro-fluoricacid, and a thin gate oxide (501) (approximately 35 Å) is grown. Thegate oxide growth is immediately followed by an in-situ doped siliconfilm. The film is heavily doped with, for example, Phosphorous for anN-type device and Boron for a P-type device. Using lithographictechniques and a silicon etch that is highly selective to oxide, thegate electrode (502) is patterned as shown in the process step (500)illustrated in FIG. 5.

3. A thin oxide (approximately 100 Å) is then thermally grown on the topsurface and sidewalls of the silicon gate electrode. Referencing FIG. 6,an anisotropic etch is then used to remove the oxide layers on thehorizontal surfaces (and thus expose the silicon (601)), whilepreserving them on the vertical surfaces. In this way, a sidewall oxide(602) is formed, and the dopants both in the gate electrode and in thechannel region of the device are electrically activated as shown in theprocess step (600) illustrated in FIG. 6.

4. Referencing FIG. 7, the final step encompasses depositing anappropriate metal (for example, Platinum for the P-type device andErbium for the N-type device) as a blanket film (approximately 400 Å) onall exposed surfaces. The wafer is then annealed for a specified time ata specified temperature (for example, 400□C for 45 minutes) so that, atall places where the metal is in direct contact with the silicon, achemical reaction takes place that converts the metal to a metalsilicide (701). The metal that was in direct contact with a non-siliconsurface (702) is left unaffected as shown in the process step (700)illustrated in FIG. 7.

5. A wet chemical etch (aqua regia for Platinum, HNO₃ for Erbium) isthen used to remove the unreacted metal while leaving the metal-silicideuntouched. The channel implanted, short channel Schottky barrier MOSdevice is now complete and ready for electrical contacting to gate,source, and drain as shown in the process step (800) illustrated in FIG.8.

This process is only one possible way to achieve channel implanted,metal source/drain Schottky MOS devices. One skilled in the art willrecognize that many other variants and alternatives exist.

Device/System

FIG. 9 shows a preferred exemplary embodiment of the invention, asexemplified by two final complementary MOSFET structures (900). Thisembodiment consists of N-channel devices fabricated with Erbium Silicide(904) for the source/drain regions, and a P-channel device fabricatedwith Platinum Silicide (905).

Vertically varying, laterally non-varying Indium (902) and Arsenic (903)layers are used as the channel dopants for the N-channel and P-channeldevices respectively. These dopant atoms are used due to theirrelatively low rates of diffusion through the silicon lattice (comparedto Phosphorous and Boron, the other two possible candidates for channeldopants). This allows for greater thermal budget during fabrication ofthe device, and therefore less statistical variation in thecharacteristics of the finished product.

The gate electrodes are fabricated from in-situ Phosphorous and Borondoped polysilicon films for the N-type (906) and P-type (907) devicesrespectively. In this instance, Phosphorous and Boron are used due totheir large solid-solubilities (compared to Arsenic and Indium). Theelectrodes are doped via the use of an in-situ method, wherein theimpurity atoms are deposited at the same time the silicon atoms are.Such a method can achieve very large doping concentrations(approximately 10²¹/cm³) and a uniform distribution throughout thethickness of the film. Another option for doping the silicon gate ision-implantation. This method suffers from several practical problemsincluding charging damage to the thin gate oxide, and the need toredistribute the highly non-uniform as-implanted gate dopants to achievehigh doping levels at the gate oxide interface.

The gate electrodes (906) and (907) are less than 100 nm in width(corresponding to the channel length L), as it is in this regime thatthe advantages of the Schottky barrier architecture over theconventional architecture become apparent. These include simplifiedprocessing due to the absence of need for pocket implants, and theresulting reduction in yield loss, capacitance and statisticalvariations in finished products.

Devices are separated from each other by a thermally grown oxide (calleda Field Oxide) (901) that works in conjunction with the channel dopantsto electrically isolate the devices from each other.

While the above description contains many specifics, these should not beconstrued as limitations on the scope of the invention, but rather as anexemplification of one preferred embodiment thereof. One skilled in theart will realize that many other variations are possible. For example,there are many possible candidates for the source/drain metal. It mayalso be advantageous to insert a thin oxide layer between the metal andthe silicon substrate. The silicon substrate itself may be replaced byany number of other semiconductors. Additionally, boundaries betweenlayers or elements can always be graded or interposed with othermaterials or interface agents to improve performance.

Generalized Process/System Fabrication

From the foregoing discussion, the process and system embodied in thepresent invention may be further generalized as illustrated in theflowcharts shown in FIGS. 10-11.

Generalized Process/System

Referencing FIG. 10, an exemplary generalized MOSFET device processfabrication flow (1000) starts with a semiconductor substrate havingmeans for electrically isolating transistors (1001). On this substrate,channel dopants are introduced in such a way that the dopantconcentration varies significantly in the vertical but not the lateraldirection (1002). Once this is complete, a gate electrode is formed(1003) on the silicon substrate. Finally, source and/or drain electrodesare formed, at least one of which incorporates a Schottky orSchottky-like contact to the semiconductor substrate (1004).

Detailed Process/System

Referencing FIG. 11, an exemplary detailed MOSFET device processfabrication flow (1100) starts with a semiconductor substrate havingmeans for electrically isolating transistors (1101). On this substrate,channel dopants are introduced in such a way that the dopantconcentration varies significantly in the vertical but not the lateraldirection (1102). Once this is complete, a gate electrode insulator isformed by growing a thin gate insulating layer and depositing aconducting film (1103) on the silicon substrate.

At this point a series of pattern and etching steps are performed toform a gate electrode (1104). Then one or more thin insulating layersare formed on one or more sidewalls of the gate electrode to selectivelyexpose the semiconductor substrate on the active areas of the deviceexcept for the sidewalls (1105). A thin film of metal is deposited onall surfaces of the device (1106), and the device is thermally annealedto form a metal-semiconductor alloy on the exposed semiconductorsurfaces (1107). Finally, unreacted metal is removed from the devicewhile leaving the metal-semiconductor alloy untouched to form localinterconnect for the formed device (1108).

Summary

In substance, the present invention may be summarized by noting that thebasic structure is one in which the substrate between the source anddrain is uniformly doped in the lateral direction, non-uniformly dopedin the vertical direction, and that the source and/or drain electrodesform Schottky or Schottky-like contacts with the substrate.

CONCLUSION

A short channel length, laterally uniform doped channel, metal sourceand drain MOS device structure and method for manufacture has beendisclosed. The present invention provides for many advantages over theprior art including lower manufacturing costs, superior devicecharacteristics, and tighter control of device parameters. Theseadvantages are achieved primarily through the introduction of a channeldoping profile that is both laterally uniform and verticallynon-uniform, in conjunction with metal source/drain regions, thuseliminating the need for halo/pocket implants and shallow source/drainextensions. The parasitic bipolar gain is unconditionally eliminated aswell.

These features of the invention make it ideal for many applications,including but not limited to high frequency and/or high radiationenvironments.

Although a preferred embodiment of the present invention has beenillustrated in the accompanying Drawings and described in the foregoingDetailed Description, it will be understood that the invention is notlimited to the embodiments disclosed, but is capable of numerousrearrangements, modifications, and substitutions without departing fromthe spirit of the invention as set forth and defined by the followingclaims.

What is claimed is:
 1. A method of manufacturing a short-channel device,for regulating the flow of electrical current, the method comprising:introducing channel dopants into a semiconductor substrate such that thedopant concentration varies significantly in the vertical direction andis generally constant in the lateral direction; providing a gateelectrode on the surface of the semiconductor substrate; and providing asource electrode and a drain electrode on the semiconductor substratesuch that the channel length is less than or equal to 100 nm, wherein atleast one of the source electrode and the drain electrode forms aSchottky or Schottky-like contact to the semiconductor substrate.
 2. Themethod of claim 1 wherein the source electrode and the drain electrodeare formed from a member of the group consisting of: Platinum Silicide,Palladium Silicide and Iridium Silicide; and further wherein the channeldopants are selected from the group consisting of: Arsenic, Phosphorous,and Antimony.
 3. The method of claim 1 wherein the source electrode andthe drain electrode are formed from a member of the group consisting ofthe rare-earth silicides, and further wherein the channel dopants areselected from the group consisting of: Boron, Indium, and Gallium. 4.The method of claim 1 wherein the gate electrode has a length notexceeding 100 nm.
 5. The method of claim 1 wherein the Schottky orSchottky-like contact formed at least in areas adjacent to the channel.6. The method of claim 1 wherein an entire surface of the at least oneof the source electrode and the drain electrode forms a Schottky orSchottky-like contact with the semiconductor substrate.
 7. The method ofclaim 1 wherein the gate electrode is provided after the completion ofall channel doping processes.
 8. The method of claim 1 wherein, when thesource and drain electrodes are provided, the substrate has a channeldopant concentration that varies significantly in the vertical directionand is generally constant in the lateral direction.
 9. The method ofclaim 1 wherein all channel dopants are introduced in one process step.10. A method of manufacturing a short-channel device, for regulating theflow of electrical current, the method comprising: introducing channeldopants into a semiconductor substrate such that the dopantconcentration varies substantially in the vertical direction and isgenerally constant in the lateral direction; providing a gate electrodeon the semiconductor substrate; exposing the semiconductor substrate onone or more areas proximal to the gate electrode; depositing a thin filmof metal; and reacting the metal with the exposed semiconductorsubstrate such that a Schottky or Schottky-like source electrode anddrain electrode are formed on the semiconductor substrate and areseparated by a channel length of no more than 100 nm.
 11. The method ofclaim 10 wherein the gate electrode is provided by the steps comprising:providing a thin insulating layer on the semiconductor substrate;depositing a thin conducting film on the insulating layer; patterningand etching the conducting film to form a gate electrode; and formingone or more thin insulating layers on one or more sidewalls of the gateelectrode.
 12. The method of claim 11 further comprising removingunreacted metal after forming the Schottky or Schotkky-like source anddrain electrodes.
 13. The method of claim 10 wherein the reacting stepis performed by thermal annealing.
 14. The method of claim 10 whereinthe source electrode and the drain electrode are formed from a member ofthe group consisting of: Platinum Silicide, Palladium Silicide andIridium Silicide; and further wherein the channel dopants are selectedfrom the group consisting of: Arsenic, Phosphorous, and Antimony. 15.The method of claim 10 wherein the source electrode and the drainelectrode are formed from a member of the group consisting of therare-earth silicides, and further wherein the channel dopants areselected from the group consisting of: Boron, Indium, and Gallium. 16.The method of claim 10 wherein the gate electrode has a length notexceeding 100 nm.
 17. The method of claim 10 wherein the source anddrain electrodes form the Schottky or Schottky-like contact with thesemiconductor substrate at least in areas adjacent to the channel. 18.The method of claim 10 wherein an entire surface of the Schottky orSchottky-like source and drain electrodes in contact with thesemiconductor substrate forms a Schottky or Schottky-like contact withthe semiconductor substrate.
 19. The method of claim 10 wherein the gateelectrode is provided after the completion of all channel dopingprocesses.
 20. The method of claim 10 wherein, when the source and drainelectrodes are provided, the substrate has a channel dopantconcentration that varies significantly in the vertical direction and isgenerally constant in the lateral direction.
 21. The method of claim 10wherein all channel dopants are introduced in one process step.
 22. Amethod of fabricating a short-channel length MOSFET device, the methodcomprising: introducing channel dopants into a semiconductor substratesuch that the dopant concentration varies significantly in the verticaldirection and is generally constant in the lateral direction; providinga gate electrode on the semiconductor substrate; and providing a sourceelectrode and a drain electrode on the semiconductor substrate such thatthe channel length is less than or equal to 100 nm, wherein at least oneof the source electrode and the drain electrode forms a Schottky orSchottky-like contact to the semiconductor substrate.
 23. The method ofclaim 22 wherein the source electrode and the drain electrode are formedfrom a member of the group consisting of: Platinum Silicide, PalladiumSilicide and Iridium Silicide; and further wherein the channel dopantsare selected from the group consisting of; Arsenic, Phosphorous, andAntimony.
 24. The method of claim 22 wherein the source electrode andthe drain electrode are formed from a member of the group consisting ofthe rare-earth silicides, and further wherein the channel dopants areselected from the group consisting of: Boron, Indium, and Gallium. 25.The method of claim 22 wherein the gate electrode has a length notexceeding 100 nm.
 26. The method of claim 22 wherein the Schottky orSchottky-like contact is formed at least in areas adjacent to thechannel.
 27. The method of claim 22 wherein an entire surface of the atleast one of the source electrode and the drain electrode forms aSchottky or Schottky-like contact with the semiconductor substrate. 28.The method of claim 22 wherein the gate electrode is provided after thecompletion of all channel doping processes.
 29. The method of claim 22wherein, when the source and drain electrodes are provided, thesubstrate has a channel dopant concentration that varies significantlyin the vertical direction and is generally constant in the lateraldirection.
 30. The method of claim 22 wherein all channel dopants areintroduced in one process step.
 31. A method of fabricating ashort-channel length MOSFET, the method comprising: introducing channeldopants into a semiconductor substrate such that the dopantconcentration varies substantially in the vertical direction and isgenerally constant in the lateral direction; providing a gate electrodeon the semiconductor substrate; exposing the semiconductor substrate onone or more areas proximal to the gate electrode; depositing a thin filmof metal; and reacting the metal with the exposed semiconductorsubstrate such that a Schottky or Schottky-like source electrode anddrain electrode are formed on the semiconductor substrate and areseparated by a channel length of less than or equal to 100 nm.
 32. Themethod of claim 31 wherein the gate electrode is provided by the stepscomprising: providing a thin insulating layer on the semiconductorsubstrate, depositing a thin conducting film on the insulating layer;patterning and etching the conducting film to form a gate electrode; andforming one or more thin insulating layers on one or more sidewalls ofthe gate electrode.
 33. The method of claim 31 further comprisingremoving unreacted metal from the MOSFET after forming the Schottky orSchottky-like source and drain electrodes.
 34. The method of claim 31wherein the reacting step is performed by thermal annealing.
 35. Themethod of claim 31 wherein the source electrode and the drain electrodeare formed from a member of the group consisting of: Platinum Silicide,Palladium Silicide and Iridium Silicide; and further wherein the channeldopants are selected from the group consisting of: Arsenic, Phosphorous,and Antimony.
 36. The method of claim 31 wherein the source electrodeand the drain electrode are formed from a member of the group consistingof the rare-earth silicides, and further wherein the channel dopants areselected from the group consisting of: Boron, lndium, and Gallium. 37.The method of claim 31 wherein the gate electrode has a length notexceeding 100 nm.
 38. The method of claim 31 wherein the source anddrain electrodes form the Schottky or Schottky-like contact with thesemiconductor substrate at least in areas adjacent to the channel. 39.The method of claim 31 wherein an entire surface of the Schottky orSchottky-like source and drain electrodes in contact with thesemiconductor substrate forms a Schottky or Schottky-like contact withthe semiconductor substrate.
 40. The method of claim 31 wherein the gateelectrode is provided after the completion of all channel dopingprocesses.
 41. The method of claim 31 wherein, when the source and drainelectrodes are provided, the substrate has a channel dopantconcentration that varies significantly in the vertical direction and isgenerally constant in the lateral direction.
 42. The method of claim 31wherein all channel dopants are introduced in one process step.